In this specification where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.
The invention relates, in general, to coatings that protect a substrate against corrosion, oxidation and metal dusting. Such protective coatings are useful, for example, in components used in chemical, petrochemical, power generation industries. Such components may include tubing, gas turbine blades and vanes, nozzles, and many other complex-shaped components, which serve in corrosive environments often at elevated temperatures. There are a variety of specially formulated coatings, such as aluminide-based coatings. These coatings may be obtained through a thermal diffusion method based on the chemical vapor deposition principles, sometimes called “pack cementation.” However, such conventional compositions, techniques, and the resulting coatings, possess a number of disadvantages and deficiencies.
In general, aluminide coatings are formed by heating of a powder mixture containing a source of aluminum (Al), an activator and an inert filler. The metallic component is immersed into this powder, and the Al-based species in a gaseous phase deposit onto the metallic substrate surface, diffuse into it and react with iron (Fe) and/or with some other metallic substrate constituents, yielding an aluminide compound, formed as a “coating” onto the substrate. These aluminides have higher corrosion and oxidation resistance, often at elevated temperatures, than the substrate material and therefore protect the components from aggressive environments.
Conventional Al-based compositions mostly contain an Al donor, an activator, and a filler. When a coating process is performed with a composition that lacks an activator, or lacks an activator and a filler, the coating formed thereby is very thin (below 25 μm or even below 15 μm), despite the use of rather high temperatures of 1050° C.-1150° C. and long soak times at such temperatures. These thin coatings are not strong enough to withstand corrosive environments when corrosive media have sufficient flows and concentrations, and the protective coating does not last an adequate amount of time.
Thus, the activator NH4CL is often used in such conventional compositions, as well as other ammonium halide activators. However, upon their decomposition at elevated temperatures, such activators form gaseous ammonia (NH3), hydrochloric acid (HCl), or other acids. These decomposition products react with aluminum, yielding aluminum chlorides or other aluminum halides, which activate the process. However, these gaseous species are hazardous to health and the environment, and they accelerate the destruction of production equipment utilized in the coating process. Thus, process economy sustainability is diminished.
In addition, such species rapidly volatize and their reaction is difficult to control in large volumes found when treating or coating larger components. Moreover, the aluminized coatings formed using such species may have a rough and uneven surface called “bisque,” with elevated contents of Al that associates with higher coating brittleness and chipping. Such coatings exhibit reduced corrosion resistance as well as reduced service life.
The use of some Al-halides as an activator, such as AlF3, AlCl3, or Na3AlF6 may be preferable to ammonium halide activators in order to avoid the formation of hazardous gases, but the coatings thicknesses (case depth) formed by such activators is often uneven and inadequate.
The parameters of the powders used for the powder mixture containing a source of aluminum (Al), an activator, and an inert filler are not well established. However, not all powders are well-suited for the above-described thermal diffusion coating processing. For instance, particle size can influence the coating process and resulting coating properties. Coarse powders are not very active for the formation of Al-halides, the coating thickness or case depth is small, and the integrity and corrosion resistance of the resulting coating may be not high enough. Fine powders are active, but they tend to form uneven agglomerates and do not have a consistent flow, resulting in rather poor and inconsistent packing with air pockets formed in the powder mixes resulting in coating micro-cracking, uneven thickness or case depth, and elevated “bisque” formation, all of which reduce the coating integrity and corrosion resistance. These effects are especially pronounced for large components to be treated and large volume production.
The Al-based coating process is conducted in high temperature furnaces, often in a protective or inert atmosphere (e.g. in argon or hydrogen) provided within the furnace. The use of furnaces with protective atmospheres is not conducive to the treatment of large products, or the treatment of many components on the same processing run. This is due to the large volume of such protective or inert gases required, making the process uneconomical and inefficient. In addition, the coating thicknesses or case depth are often not large enough. An increase in process temperature and time may increase the case depth, however, this is not desirable because of the steels and alloys of the treated components or substrates can be degraded by elevated temperatures and soak times. For instance, exposure to elevated treatment temperatures can result in elevated migration of chromium or other alloying elements to the surface and around the grains, and possible depletion that makes the metal structure uneven and less ductile.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass or include one or more of the conventional technical aspects discussed herein.